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EP-3671117-B1 - COMPENSATION FOR MAGNET AGING IN INDUCTIVE GYROSCOPE

EP3671117B1EP 3671117 B1EP3671117 B1EP 3671117B1EP-3671117-B1

Inventors

  • WILLIAMSON, MATTHEW
  • SHEARD, JOHN KEITH
  • GREGORY, CHRIS

Dates

Publication Date
20260506
Application Date
20191127

Claims (10)

  1. A vibrating structure gyroscope (30), comprising: a permanent magnet (22); a structure (10) arranged in a magnetic field of the permanent magnet (22) and arranged to vibrate under stimulation from at least one primary drive electrode; a drive system (31) arranged to vibrate the structure (10) at a resonance frequency, the drive system (31) comprising: the at least one primary drive electrode arranged to induce motion in the vibrating structure (10), at least one primary sense electrode arranged to sense motion in the vibrating structure (10); and a drive control loop controlling the primary drive electrode dependent on the primary sense electrode; the vibrating structure gyroscope being characterized by a compensation unit (36) arranged to receive a signal from the drive system (31) representative of a gain in the drive control loop and arranged to output a scale factor correction (37) for application to a rate signal of the gyroscope (30) based on that signal and a stored reference value of said signal obtained during a calibration procedure, and further based on a known relationship between said signal, a magnetic field strength and a scale factor error.
  2. A vibrating structure gyroscope (30) as claimed in claim 1, wherein the signal from the drive system (31) comprises one or more of: the amplitude of the drive signal for the primary drive electrode; the amplitude of the signal from the primary sense electrode; the gain of the drive control loop.
  3. A vibrating structure gyroscope (30) as claimed in any preceding claim, wherein the compensation unit (36) includes a lookup table (38) that is arranged to provide a scale factor correction (37) value according to the input signal from the drive system (31).
  4. A vibrating structure gyroscope (10) as claimed in any preceding claim, wherein the compensation unit (36) is arranged to receive a temperature signal and to output a scale factor correction (37) based on both the signal from the drive system (31) and the temperature signal.
  5. A vibrating structure gyroscope (30) as claimed in claim 4, wherein the compensation unit (36) includes a lookup table (38) that is arranged to provide a scale factor correction (37) value according to both the signal from the drive system (31) and the temperature signal.
  6. A vibrating structure gyroscope (30) as claimed in any preceding claim, further comprising: a sensing system arranged to sense the vibrations of the vibrating structure and arranged to output an angular rate signal based on the sensed vibrations; and wherein the vibrating structure gyroscope (30) is arranged to apply the scale factor correction (37) to the angular rate signal to provide an output of the vibrating structure gyroscope (30).
  7. A method of calibrating a gyroscope, comprising: providing a gyroscope (30) as claimed in any preceding claim; evaluating the strength of the signal from the drive system (31) in a test environment while the gyroscope (30) is not rotating; and storing in the compensation unit (36) information based on that evaluation that allows determination of the scale factor correction (37) from the signal from the drive system (31).
  8. The method of claim 7, further comprising storing in the compensation unit (36) information on the relationship between the strength of the signal from the drive system (31) and the strength of the magnetic field of the permanent magnet (22).
  9. A method as claimed in claim 7 or 8, wherein the step of evaluating comprises evaluating the strength of the signal from the drive system (36) across a range of temperatures.
  10. A method as claimed in claim 7, 8 or 9, wherein the step of storing comprises storing the information in a lookup table (38).

Description

Technical Field The present disclosure relates to vibrating structure gyroscopes, particularly Microelectromechanical Systems (MEMS)-based vibrating structure gyroscopes for the measurement of angular rate(s), e.g. in inertial measurement units (IMUs). The present disclosure is concerned in particular with inductive gyroscopes. WO2010/007406 describes an inductive-type vibrating structure gyroscope comprising primary drive and primary pick off transducers controlled using an Automatic Gain Control loop, and secondary drive and pick off transducers used to detect an applied rotation. US2017/343351 describes a MEMS gyroscope comprising a mass element, a drive actuator, sense electrodes, and a control unit, in which the mass element is driven to oscillate by the drive oscillator, and a sensed signal is detected using the sense electrodes. US7801694 describes a method for compensating for bias and scale factor errors in piezoelectric gyro resonators. Background Gyroscopes are sensors that measure angular rate (i.e. the rate of rotation). Gyroscopes are used in many applications, including inertial navigation, robotics, avionics, and automobiles. In inertial navigation applications, gyroscopes may be found in self-contained systems known as "inertial measurement units" (IMUs). IMUs typically contain a plurality of accelerometers and/or gyroscopes, and provide an estimate of an object's travel parameters such as angular rate, acceleration, attitude, position, and velocity, based on the outputs of gyroscope(s) and/or accelerometer(s). MEMS-based gyroscopes have become ubiquitous in recent years, and are often far more effective than their conventional macroscopic counterparts. MEMS-based gyroscopes are typically implemented using vibrating structures and are often referred to in the art as "vibrating structure gyroscopes" or "VSGs". Vibrating structure gyroscopes generally contain a micro-machined mass that is arranged to oscillate. Typical examples of vibrating structure gyroscopes include vibrating ring gyroscopes, vibrating tuning fork gyroscopes, and also other vibrating structures including e.g. beams, cylinders, hemispherical shells, and disks. In general operation, the micro-machined mass is driven to oscillate in a predefined mode of vibration, typically a cos nθ mode of vibration (e.g. n = 2). The driven mode of vibration is usually called a primary mode. When the gyroscope rotates, a Coriolis force is exerted on the vibrating mass, and this force may cause the mass to oscillate in a secondary mode of vibration, which is different to the primary mode. Typically, the secondary mode of vibration occurs in addition to the primary mode, and the secondary mode results in the mass oscillating along a different direction to the predefined oscillation of the primary mode. Since the amplitude of oscillation in the secondary mode is proportional to the rate of rotation, the angular rate (e.g. measured in degrees per second) can be determined by directly detecting the amplitude of the secondary oscillation using a suitable sensor (e.g. a transducer such as an inductive or capacitive transducer) - this is known as an "open loop measurement". Alternatively, the angular rate may be measured by applying a restorative force to counter the oscillation in the secondary mode and thereby keep the mass vibrating solely in the primary mode. The restorative force is usually based on the detected amplitude of the secondary oscillation. Since the restorative force is proportional to the applied angular rate, the amplitude of the signal required to nullify the secondary mode provides a measure of the angular rate. This latter arrangement is known in the art as a "closed loop measurement". An example of how to measure the angular rate is discussed in, for example, US 5,419,194 and US 8,347,718. An issue with vibrating structure gyroscopes that use permanent magnets is that the performance of the gyroscope can degrade as the magnet deteriorates over time. Inductive (as opposed to capacitive or piezoelectric) gyroscopes use a permanent magnet to create a magnetic field as part of their operation. One example of an inductive gyroscope structure is shown in Fig. 1. The inductive gyroscope 1 comprises a lower pole piece 20, an upper pole piece 24 and a permanent magnet 22 sandwiched between them. The vibrating ring 10 is located between the upper pole piece 24 and the lower pole piece 20 such that it lies within the magnetic field formed between these two pieces. The vibrating ring 10 is mounted via external mounting legs (not shown) that extend from the radially outer edge of the ring 10 to the support frame 12 such that it is able to vibrate as described above. The support frame 12 is typically mounted to a glass pedestal 14 which in turn is typically mounted upon a glass substrate 16. In use, the lines of magnetic flux pass through the gyroscope structure (i.e. the ring 10). Conductive tracks are formed on the gyroscope structure (no